1. Field of the Invention
The present invention relates to a device for controlling brakes of a vehicle such as an automobile, and more specifically, to such a device that controls braking force distribution among front and rear wheels in a vehicle.
2. Description of Prior Art
During braking of a vehicle, a rear wheel is liable to be locked because its frictional circle is shrunk due to the forward shifting of the load of the vehicle. The locking of a rear wheel, prior to a front wheel, induces serious deterioration of a vehicle running behavior, such as disturbance in the attitude and/or spinning of a vehicle body. In order to avoid the locking of a rear wheel, braking force distribution (BFD) control has been proposed to keep braking force on rear wheels lower than on the front wheels. In such BFD control, braking pressures applied to rear wheel cylinders in a hydraulic braking system is held, reduced or pulsatively increased, i.e. the increase in braking force generated on rear wheels is restricted, providing a distribution of braking force biased to front wheels. Usually, BFD control, often referred to as “Electronic Braking force Distribution Control (EBD control)”, is executed by a computerized device operating a plurality of solenoid valves in a hydraulic circuit. Examples of devices executing EBD control are seen in Japanese Laid-Open Patent Publications (JP) Nos. 5-213169 and 2001-219834.
By the way, the restriction of rear wheel braking force in BFD control is to be started when the braking force or braking pressure of the rear wheel exceeds an appropriately determined value. In some of the conventional BFD control devices, however, BFD control is not always started at an appropriate time due to several constraints involved in a physical structure of a braking system, especially when a braking action of a driver of a vehicle, such as the depression of a brake pedal, is fast.
For instance, in some control devices for a hydraulic braking system where no wheel cylinder pressure sensor is available, the BFD control is started in response to a master cylinder pressure increase on the assumption that the pressures in wheel cylinders is equal to a master cylinder pressure in absence of any special control operation. However, the pressure variations in wheel cylinders are delayed relative to the master cylinder because the transmission of a pressure variation in a master cylinder to wheel cylinders takes a certain time dependent upon the length and physical structures of piping between the master and wheel cylinders. The delay of the pressure variations in wheel cylinders, especially upon a rapid depression of a brake pedal, would cause a premature holding of the wheel cylinder at a pressure lower than to be held by closing the corresponding valves before the pressure in the master cylinder is fully reflected in the wheel cylinders.
Further, in some of the conventional BFD control devices, BFD control is started in response to the increase of deceleration of a vehicle, where a signal of the deceleration, detected with a sensor, is filtered through a low-pass filter having a predetermined cut-off frequency for eliminating noise components from the deceleration signal. Upon a rapid depression of a brake pedal, however, a higher frequency component (reflecting a rapid variation) in the deceleration signal is also eliminated by the low-pass filtering, resulting in delay of the detection of the actual deceleration increase and thus the starting of BFD control.
Accordingly, a BFD control device should be improved with respect to the accuracy in the timing of starting of the control.